Gas-generating pyrotechnic products

ABSTRACT

The subject of the present invention is a pyrotechnic gas-generating product, the composition of which comprises: 
     guanidine nitrate, 
     basic copper nitrate, and 
     potassium perchlorate. 
     Characteristically, said potassium perchlorate represents between 8% and 20% of the total weight of said pyrotechnic product and said composition additionally contains at least one oxide, chosen from metal oxides, metalloid oxides and mixtures thereof; said at least one oxide having a melting point below the combustion temperature of said pyrotechnic product and said at least one oxide representing between 1% and 5% of the total weight of said pyrotechnic product. Said at least one oxide improves the combustion, at low pressure, of said pyrotechnic product.

The present invention relates to pyrotechnic gas-generating products,suitable for use in motor vehicle occupant protection systems, forexample for the inflation of airbags or for seatbelt pretensioners.

The technical field relating to motor vehicle occupant protection hasexperienced a very large expansion over the last twenty years. Thelatest generation vehicles from now on integrate within the passengercompartment several safety systems, of airbag type or of seatbeltpretensioner type, the operation of which is carried out by thecombustion gases of pyrotechnic products. Among the airbag-type systems,airbags for front impact (driver or passenger airbags) and those forside impact (curtain or thorax-protection airbags) are mainlydistinguished.

In view of the required reductions in the cost of gas generators forairbags imposed by motor vehicle manufacturers, the pyrotechnic chargemust simultaneously satisfy the following requirements:

-   -   the gases generated by the combustion of the pyrotechnic charge        must be non-toxic, that is to say have a low content of carbon        monoxide, of nitrogen oxides and of chlorinated compounds;    -   the gas yield of the composition (that is to say the amount of        gas generated by the combustion) must be high in order to lead        to a high inflation power. This is given by the product of the        molar gas yield of said composition (in mol/kg) and its        combustion temperature Tc (in K);    -   in a correlated manner, the amount of solid particles generated        by the combustion, capable of constituting hot spots that may        damage the wall of the airbag, must remain low;    -   the combustion temperature must not be too high (ideally less        than 2200 K) so that the temperature of the gases in the airbag        is low enough to not attack the physical integrity of the        occupant. A low combustion temperature makes it possible, on the        one hand, to limit the thickness of the bag and, on the other        hand, to simplify the design of the gas generator by making it        possible to reduce the presence of baffles and filters within        this bag. Finally, the gas generator has a reduced weight and        reduced volume, at a lower cost; and    -   the pyrotechnic composition must have a high value of the        inflation rate per unit area, which rate is estimated by the        product ρ×n×Tc×Vc, where ρ is the density of the pyrotechnic        material (expressed in g/cm³), n is the molar gas yield of the        combustion (expressed in mol/g), Tc is the combustion        temperature (expressed in Kelvin) and Vc is the combustion rate        (expressed in mm/s). Thus, the parameter of inflation rate per        unit area is expressed in mol.K.mm⁻².s⁻¹.

The airbag systems for side application differ from those for frontapplication essentially due to the time required for the deployment andpositioning of the airbag. Typically, this time is shorter for a sideairbag (of the order of 10-20 ms, compared to 40-50 ms for a frontairbag). For a side airbag, the functional requirement of inflation ofthe bag over a short time makes it necessary to resort to a pyrotechniccomposition having a high combustion rate (typically greater than 35mm/s at 20 MPa and greater than 40 mm/s at 50 MPa) over the entireoperating pressure range in the combustion chamber of the generator(typically of the order of 20-80 MPa), in order to obtain a sufficientvalue of the inflation rate per unit area (product ρ×n×Tc×Vc).Furthermore, in order to ensure a satisfactory start-up of the system,the pyrotechnic composition must also have good ignitabilitycharacteristics. Lastly, considering the generally tapered surfaceprofile of the charges used (of pellet type), the composition shouldideally have a combustion rate that is stable and high enough at lowpressure (low pressure is understood to mean a pressure equal to orslightly greater than atmospheric pressure).

Furthermore, the airbag systems for side application may require two gasgenerator technologies: those which are said to be entirely pyrotechnic(the gas generation then being carried out exclusively by the combustionof a pyrotechnic charge) and those said to be “hybrid” (the gases thenoriginating jointly from the combustion of a pyrotechnic charge and froman inert gas volume stored under pressure in a leaktight reservoir). For“hybrid” generators, the pyrotechnic charge must not have too low acombustion temperature so that the combustion gases are hot enough tocompensate for the drop in temperature generated by the volume expansionof the precompressed inert gas. Ideally, combustion temperatures above2000 K are required.

Thus, a person skilled in the art is in search of pyrotechniccompositions that simultaneously have a moderate combustion temperatureof the order of 2000-2200 K and a high combustion rate over the entireoperating pressure range, including at low pressure, so that saidcompositions are suitable for use in entirely pyrotechnic gas generatorsor in hybrid generators intended for side airbags.

Various types of pyrotechnic compositions have already been proposed todate. Currently, the pyrotechnic compositions that appear to offer thebest compromise in terms of combustion temperature and toxicity of thecombustion gases are formulated from the mixture of basic copper nitrate(BCN) as oxidizing charge and guanidine nitrate (GN) as reducing charge.The use of the BCN/GN pair makes it possible to obtain a low combustiontemperature, typically of the order of 1800 K. Patent U.S. Pat. No.5,608,183 describes such compositions, obtained by a wet manufacturingprocess. However, these compositions have, as major drawbacks:

-   -   a high content of non-filterable solid residues. These residues        originate from the decomposition of the BCN, in the form of        droplets of liquid copper at the temperature that exists in the        combustion chamber of the gas generator, droplets which solidify        on leaving said generator. The resulting hot solid particles are        then capable of damaging the wall of the airbag;    -   a difficult “ignitability” requiring the use of a sizeable        ignition charge, which increases the cost of the generator;    -   a gas yield that is not very high;    -   a too low combustion temperature, which makes it difficult to        render their use compatible in “hybrid” generators; and    -   a low combustion rate (close to 20 mm/s at 20 MPa), which makes        their use in entirely pyrotechnic generators or “hybrid”        generators for side airbags difficult.

According to the prior art, it has been proposed, in order to overcomethe first of the major drawbacks mentioned above, to incorporate, intothe pyrotechnic composition, a refractory oxide such as aluminum oxideor silicon oxide for the purpose of agglomerating the liquid combustionresidues of the BCN. Thus, patent U.S. Pat. No. 6,143,102 describescompositions based on BCN and on GN, still obtained by the wet route,incorporating, as refractory oxide, silicon oxide, at a content whichmay range up to 5% by weight. A person skilled in the art knows thatthis agglomeration effect is made possible by the fact that siliconoxide has a melting point (or softening point) of 1950 K, greater than,or at the very least close to, the combustion temperature of thecomposition Tc=1800 K, so that the softened solid-state oxide makes itpossible to agglomerate the droplets of liquid copper. Thus, at the endof the combustion, the backbone of the pyrotechnic block is obtained.However, the incorporation, even at a low content, of such anagglomerating oxide rapidly proves prejudicial to the combustion rate,due to the fact that this agglomeration effect generates a particulategangue which remains in contact with the pyrotechnic block (of thepellet) during the combustion and which limits the heat flow to thesurface not yet burnt. This type of pyrotechnic composition thereforehas the drawback of a low combustion rate and a low gas yield. Tocompensate for said low combustion rate of the compositions described,the incorporation is proposed, in patent U.S. Pat. No. 6,143,102, of asecond refractory additive of metal oxide type (aluminum, titanium,zirconium, zinc or magnesium oxide) as ballistic catalyst. Finally, theincorporation of silicon oxide and of a metal oxide, at a total contentclose to 10% (by weight), is greatly detrimental to the gas yield valueof the composition.

To improve the “ignitability” of compositions based on BCN and on GN,the addition of potassium perchlorate to these compositions has beenproposed according to the prior art. Patent application EP 1 526 121thus describes the addition of a perchlorate (in particular potassiumperchlorate), at a low content (less than 5% by weight), in order toimprove the ignition of said compositions. The incorporation ofperchlorate at such a low content makes it possible to slightly increasethe combustion rate and the gas yield of the composition, thisimprovement remaining however insufficient for use in gas generators forside airbags.

Patent application US 2006/0016529 describes compositions based onguanidine nitrate (40% to 60% by weight), on basic copper nitrate (35%to 50% by weight), on alkali metal perchlorate, present at contentswhich may be higher than those according to the teaching of patent EP 1526 121 (1% to 10% by weight) but remaining limited, and on metal oxides(1% to 5% by weight) playing the part of ballistic catalyst andagglomerating agent. Said metal oxides are present for the sameagglomeration purposes as according to the teaching of patent U.S. Pat.No. 6,143,102 (see above).

The addition of potassium perchlorate in a larger amount (in an amountthat is however limited, that is to say typically less than 30%, so asnot to lead to an unacceptable rise in the combustion temperature)results in a notable increase in the gas yield, and also, when it isassociated with a particular process for obtaining the product, in anotable increase in the combustion rate. Thus, patent application FR 2892 117 describes a composition based on guanidine nitrate (reducingagent), on basic copper nitrate (main oxidizing agent) at a reducedcontent and on potassium perchlorate (co-oxidizing agent) at a highercontent, up to 30% by weight. The low content of basic copper nitratedoes not require the addition of agglomerating agent to the composition(the small amount of copper particles produced by the BCN beingacceptable within the context of the application described), and thehigh content of potassium perchlorate associated with a specific processfor obtaining the product makes it possible to achieve high combustionrates at medium and high pressure that do not require the addition ofballistic catalyst. These compositions constitute the prior art closestto the present invention.

Patent application FR 2 892 117 therefore teaches that the combinationof moderate contents of potassium perchlorate (close to 14%), of basiccopper nitrate and of a use of a dry compacting process results inpyrotechnic compositions being obtained that advantageously reconcile:

-   -   a combustion temperature close to 2100 K,    -   a high combustion rate at high pressure,    -   a high gas yield,    -   a good “ignitability”, and    -   a small quantity of solid (copper) particles generated during        the combustion, which makes it possible to do away with the        presence of an agglomerating agent.

However, this type of composition has a combustion pressure limit whichis located above atmospheric pressure. The absence of self-sustainingcombustion at atmospheric pressure, and also a high pressure exponentbelow 2 MPa, may result, depending on the operating pressure and thegeometry of the charge, in extinguishment at the end of combustion.Indeed, a person skilled in the art knows the impact induced by theincorporation of perchlorate, which proves favorable to the combustionrate at high pressure but less favorable to the combustion at very lowpressure, once the content of perchlorate incorporated becomes large. Atlow pressure, the high expansion generated by the high gas yieldassociated with the low content of solid particles induces a slightreturn of heat flow to the unburnt zone: this being the case, thecombustion is self-sustaining with difficulty.

For a pyrotechnic composition, the fact of not having a stable andself-sustaining combustion at low pressure constitutes a major drawbackwhen said composition is used in a gas generator for airbags, mainly forthe following reasons:

-   -   risk of extinguishment at very low pressure at the beginning or        end of operation, due to a high value of the pressure exponent,        which may require resorting to a co-charge in order to maintain        the combustion of the main charge at low pressure. Thus, the gas        generator is bulkier, less compact and therefore more expensive;    -   risk of extinguishment at the end of operation (when the        pressure in the chamber of the generator drops below the        self-sustaining combustion pressure limit of the pyrotechnic        composition), extinguishment that results in the presence of        unburnt products, which do not contribute to the generation of        combustion gas participating in the inflation of the bag        according to the objective functional requirement. Furthermore,        these unburnt products may gradually be degraded via a pyrolysis        phenomenon under the effect of a residual high temperature in        the combustion chamber. This slow degradation via pyrolysis        results in the emission of bursts of gas that are difficult to        control and sometimes solid particles of small size which cannot        be captured by the filter. Such a phenomenon results in the        appearance of fumes at the end of operation, which are        prejudicial to compliance with the standards of toxicity and of        emission of breathable particles that are in force in the field        (USCAR).

In such a context, the inventors desired to propose improved pyrotechnicgas-generating products, which are improved in that they simultaneouslymeet the following objectives:

a moderate combustion temperature (close to 2100 K),

a high gas yield (greater than 30 mol/kg),

a limited content of solid particles generated during combustion,

a good “ignitability”,

a high combustion rate at high pressure (greater than 35 mm/s at 20 MPa,greater than 40 mm/s at 50 MPa), and

a combustion, with advantageous combustion rates, that is stable andself-sustaining at low pressure, ideally at atmospheric pressure, makingit possible to avoid a risk of extinguishment of the charge duringoperation in generator.

Within the context of the present invention, the inventors have shown,more particularly with reference to the technical problem of improvingthe combustion at low pressure while maintaining a high combustion rateat high pressure, the great advantage in incorporating, in a limitedcontent (from 1% to 5% by weight), in a composition containing amoderate amount of potassium perchlorate, at least one oxide, chosenfrom metal oxides, metalloid oxides and mixtures thereof, said at leastone oxide having a melting point below the combustion temperature of thepyrotechnic product (in order to thus avoid any agglomeration effect ofthe combustion residues, which is prejudicial to maintaining asufficiently high combustion rate at high pressure). Said at least oneoxide, for the formation of a homogeneous pulverulent mixture(comprising mainly GN+BCN+KClO₄+said at least one oxide (see below))intended to be used with a view to the formation of a pyrotechnicproduct of the invention via dry route, is in the form of a pulverulentcharge of micron-scale particle size (typically between 0.1 and 100 μm)and/or of high specific surface area (>20 m²/g). These are customarycharacteristics for a constituent of this type.

The inventors therefore presently propose novel high-performancepyrotechnic products, for use in gas generators of “hybrid” type or ofentirely pyrotechnic type, which are particularly suitable for use inairbag systems for side application.

The compositions of the pyrotechnic gas-generating products of theinvention (very particularly suitable for airbag applications) comprise:

guanidine nitrate (as nitrogen-containing reducing charge),

basic copper nitrate (as main oxidizing charge), and

potassium perchlorate (as secondary oxidizing charge).

Characteristically: said potassium perchlorate represents between 8% and20%, advantageously between 10.5% and 20%, of the total weight of saidpyrotechnic product; and

-   -   said composition additionally comprises at least one oxide,        chosen from metal oxides, metalloid oxides and mixtures thereof;        said at least one oxide representing between 1% and 5% of the        total weight of said pyrotechnic product and having a melting        point below the combustion temperature of the pyrotechnic        product.

The ingredients of the four types above (GN, BCN, KClO₄ and oxide(s) ofthe aforementioned type) generally represent more than 90% by weight ofthe composition of the products of the invention, very generally morethan 95%, or more than 98%, or even 100%, by weight. The optionalpresence of additives, such as manufacturing auxiliaries (for examplecalcium stearate), is expressly anticipated.

Guanidine nitrate was chosen as a reducing agent for its thermodynamicproperties (especially its gas yield), for the ballistic properties thatit imparts to the pyrotechnic product, and for its rheoplastic behaviorfavorable to the use of the dry process for obtaining said pyrotechnicproduct. Said guanidine nitrate is particularly advantageous forpyrotechnic safety reasons and for this rheoplastic behavior highlysuited to the use of compacting and optional pelletizing phases of thedry process, ensuring a good densification of the pyrotechniccomposition while limiting the compressive stress to be applied. Themanufacture of pyrotechnic products via the dry process generallycomprises four main steps, which have in particular been described inpatent application WO 2006/134311.

The potassium perchlorate is present, in the composition of the productsof the invention, in a moderate content (from 8% to 20% by weight), veryparticularly with reference to the combustion temperature, the“ignitability” and the combustion rate at high pressure that aretargeted. It is advantageously present for at least 10.5% by weight.

It has been understood that the function of said at least one metaland/or metalloid oxide is not, as in the prior art (see in particularthe teachings of patent U.S. Pat. No. 6,143,102 and application US2006/0016529 recalled above), to agglomerate the liquid copper particlesduring the combustion in order to form, during said combustion, aparticulate gangue prejudicial to obtaining a high combustion rate athigh pressure, but to ensure within a composition (containing a moderatecontent of KClO₄), the combustion temperature of which is greater thanthe melting point of said at least one metal and/or metalloid oxide,surprisingly:

-   -   a stable and self-sustaining combustion at lower pressure than        that of the compositions of the prior art,    -   a combustion rate at low pressure that is higher than that of        the compositions of the prior art;        the potassium perchlorate present within said composition being,        for its part, essentially responsible for    -   a high combustion rate at high pressure, close to that of the        compositions of the prior art, and    -   a pressure exponent over the whole of the pressure range equal        to or lower than that of the compositions of the prior art.

In this way, the products of the present invention advantageouslyreconcile:

a moderate combustion temperature (close to 2100 K),

a high gas yield,

a high combustion rate at high pressure, and

a non-zero combustion rate at low pressure, or even at atmosphericpressure,

with a “good” “ignitability” and without generating too many solidparticles during the combustion.

Within the context of the present invention, an original use of metaland/or metalloid oxides (known as ballistic catalysts and/oragglomerating agents) is therefore proposed for improving the combustionat low pressure (see above).

Said at least one oxide, present in the composition of the products ofthe invention at at least 1% by weight, is therefore responsible for animprovement in the combustion at low pressure. Its content is limited to5% by weight, with reference, very particularly, to the gas yield and tothe combustion at high pressure of said products.

In order to obtain the products of the invention, the necessaryproportions of the aforementioned ingredients (constituents of saidproducts) resulting in the desired properties (those of said propertieswhich result from thermodynamic calculations: combustion temperature,gas yield, content of solid residues, oxygen balance and theoreticaldensity, etc.) are therefore determined beforehand while ensuring thatthe condition relating to the melting point of the at least one metaland/or metalloid oxide present (melting point which must be lower thanthe combustion temperature of the product containing, in itscomposition, said at least one oxide) is met (then, the advantageousresults, demonstrated by the inventors, are observed over combustion atlow pressure).

The melting point of said at least one oxide present (of the oxidepresent or of each of the oxides present) in the composition of thepyrotechnic products of the invention is advantageously at least 50 Kbelow the combustion temperature of said pyrotechnic product.

Preferably, said at least one oxide is chosen from silicon oxide (SiO₂),tungsten oxide (WO₃) and molybdenum oxide (MoO₃). Silicon oxide (SiO₂)is more particularly preferred.

Advantageously, the compositions of the products of the inventioncomprise, expressed as weight percentage:

-   -   from 50% to 65% of guanidine nitrate,    -   from 20% to 40% of basic copper nitrate,    -   from 8% to 20%, advantageously from 10.5% to 20%, of potassium        perchlorate, and    -   from 1% to 5%, advantageously from 1% to 3%, of said at least        one metal and/or metalloid oxide.

The products of the invention are therefore very advantageously obtainedby a dry manufacturing process which comprises a first step of drymixing the ingredients in powder form and a second step of compactingthe pulverulent mixture obtained. These two steps are optionallyfollowed by a third step of granulation, itself followed, if necessary,by a fourth step of forming, via pelletizing, of the granules obtainedin order to obtain compressed products.

The products of the invention are therefore generally in the form ofgranules, pellets or blocks.

So as to facilitate the implementation of the pelletizing, byinterfering as little as possible with the desired functionalperformances of the final product, an additive may be added, after thegranulation phase. This additive is advantageously from the family ofstearates. It preferably consists of calcium or magnesium stearate. Thecontent added is less than 1% and preferably less than 0.5% (% byweight).

The products of the invention are very particularly suitable for beingintegrated into the pyrotechnic charge of a gas generator for airbags.They may constitute all or part of said charge.

According to another of its subject matters, the present inventionrelates to gas generators containing at least one pyrotechnic product ofthe invention. Said generators are perfectly suitable for airbags (seeabove).

It is now proposed to illustrate, in no way limitingly, the inventionpresently claimed. Compositions (of products of the invention)illustrating several variants of the invention are described andcompared with examples of compositions (of products) from the prior art.

FIG. 1 shows the curves of rate of (propagation of) combustion at lowpressure for products of the invention and products from the prior art.Measurements were performed on the granules via the “strand burner”technique (see below).

FIG. 2 shows the curves of combustion rate over a wide pressure rangefor products of the invention and products from the prior art.Measurements were performed on the pellets in a manometric bomb.

Table 1 below presents examples of compositions of products of thepresent invention, and also their associated performances. Thecompositions (products) were evaluated by means of thermodynamiccalculations or from physical measurements obtained on granules orpellets manufactured from said compositions via the dry process ofpowder mixing—compacting—granulation—and optionally pelletizing.

As a function of the content of oxide incorporated, the content of themajor constituents (GN, BCN and KClO₄) was adjusted in order to maintainan oxygen balance value close to −3%, so as to be able to directlycompare the performances of the compositions from table 1.

The major constituents used in the compositions described in table 1advantageously have a fine particle size, characterized by a value ofthe median diameter (D50) of around 12 μm for the guanidine nitrate, ofaround 3 μm for the BCN and of around 10 μm for the KClO₄.

The metal or metalloid oxides used in the compositions of examples 1 to4 are characterized by a melting point of around 1950 K (SiO₂), 1070 K(MoO₃) and 1750 K (WO₃). The silicon oxide has a specific surface areaof 100 to 200 m²/g, the molybdenum oxide has a median diameter centeredabout 10 μm and the tungsten oxide has a median diameter centered about100 μm.

The compositions of examples 1 to 4 are constitutive of productsaccording to the present invention, those of the reference examples(comparative examples A and B) are constitutive of products according topatent application FR 2 892 117 from the prior art.

TABLE 1 Ref. A Ref. B example 1 example 2 example 3 example 4 % byweight Composition of the products KClO₄ 10 8 11.8 11.6 11.7 11.7guanidine nitrate 58.2 57.2 58.2 57.3 57.6 57.6 basic copper nitrate21.8 34.8 28.5 28.1 28.2 28.2 SiO₂ 0 0  1.5 3 0 0 MoO₃ 0 0 0  0  2.5 0WO₃ 0 0 0  0  0 2.5 Thermodynamic calculations Combustion temperature2068 2041 2077    2059    2065 2077 at 20 MPa (K) Gas yield at 1 bar and30.9 30.7 30.7 30.2 30.4 30.4 1000 K (mol/kg) Content of solid 22.9 23.323.6 24.7 22.2 24.3 residues at 1 bar and 1000 K (%) oxygen balance (%)−3.1 −3.1 −3.1 −3.0 −3.0 −3.0 theoretical density 1.869 1.889   1.858  1.866 1.878 1.884 (g/cm³) Ballistic performances measured Lowcombustion 5 3.5   <1 (*)   <1 (*) 1.5 2.5 pressure limit (bar)Combustion rate at 0 0  0.6  1.1 0 0 atmospheric pressure (mm/s)Combustion rate at 39.5 36.6 39.2 36.4 35.7 35.7 20 MPa (mm/s)Combustion rate at 48.6 46.6 44.8 42.0 44.4 45.0 50 MPa (mm/s) pressureexponent 0.21 0.24  0.13  0.13 0.23 0.24 Appearance of the noagglomerated residues in the form of a backbone combustion residues ofthe pyrotechnic block Mechanical strength and densification on pellets(**) Porosity (%) 1.1 1.3 <1   <1   1 0.9 Radial crushing strength 15.815.3 16.4 17.6 16.2 16.5 (kP) (*) non-zero combustion propagation rateat atmospheric pressure (**) pellets having a diameter of 6.35 mm and athickness of 3 mm

For each of the products from table 1, the combustion pressure limit wasmeasured on granules via the “strand burner” technique (firings in apressurized vessel). For this, the granules are introduced into a strawhaving a diameter of 7.4 mm, which is placed in a chamber having acapacity of 5 liters that is pressurized under an inert atmosphere (N₂).The ignition is carried out using a hot wire, then the measurement ofthe rate of propagation of the combustion is performed using 2 fusiblewires lodged in the straw and spaced 100 mm apart. The firings werecarried out at 20° C. for various vessel pressurization values until thenon-ignition of the granules of each composition was observed.

For each of the products from table 1, the combustion rate (Vc) wasmeasured on pellets by means of firings carried out in a manometricbomb. The firings were carried out for various charge density values (35kg/m³ to 175 kg/m³) in order to establish the curve Vc(P) over a widepressure range.

The results from table 1 indicate that the compositions of examples 1 to4 according to the invention have:

-   -   advantageously, a maintenance of the performances, in terms of        density, combustion temperature and gas yield, compared to the        compositions of comparative examples A and B. These performances        are important because they play a major part in the expected        function of the composition (inflation power);    -   a good aptitude for densification, as the low porosity values        measured on pellets indicate. This aptitude for densification is        important for the manufacture of compacted granules and also for        the manufacture of pellets, the geometry, diameter or thickness        of which may be easily adapted depending on the envisaged        application. It also makes it possible to be able to apply a        minimum compressive stress during the processing of the product        via pelletizing, which reduces wear of the tools and the        pyrotechnic risks during compression; and    -   result in pellets being obtained that have a satisfactory        mechanical strength. The incorporation, at a moderate content,        of an oxide of SiO₂, MoO₃ or WO₃ type does not degrade the        mechanical strength of the pellets, as the radial crushing        strength values indicate.

The low combustion pressure limit values indicated in table 1 and thecombustion rate curves at low pressure from graph 1 show that theincorporation, at a moderate content (between 1.5% and 3% in theexamples), of an oxide of silicon oxide, molybdenum oxide or tungstenoxide type makes it possible to significantly decrease the combustionpressure limit value relative to that of the reference compositions Aand B. Among the compositions of examples 1 to 4, the composition ofexample 2 formulated with 3% of silicon oxide has the most notableimprovement since it has a non-zero combustion rate value at atmosphericpressure and, generally, the highest combustion rate over the pressurerange extending from 0.1 to 1 MPa.

With reference to FIG. 2, the ballistic characterization in a manometricbomb carried out on pellets manufactured from compositions cited intable 1 show that the compositions of examples 1 to 4 have a combustionrate that remains high enough over the high-pressure range extendingfrom 20 to 50 MPa.

The product of example 1, formulated with 1.5% of silicon oxide, offersthe best compromise of performances between combustion temperature, gasyield, combustion pressure limit and combustion rate at high pressure.This product especially has the great advantage of maintaining anon-zero combustion rate at atmospheric pressure. The incorporation ofsilicon oxide at a content of 3% according to the composition of example2 appears even more beneficial to combustion at very low pressure, butin return generates a reduction in the combustion rate at high pressure.These results indicate that the silicon oxide is advantageouslyincorporated up to a content of 3% in order to preserve a sufficientballistic performance at high pressure over the range 20 to 50 MPa.

Among the various oxides tested, a beneficial reduction of the pressureexponent was observed for the products of examples 1 and 2, formulatedaccording to the present invention with silicon oxide.

Due to the fact that the melting point of the oxide incorporated (SiO₂,MoO₃ or WO₃) remains below the combustion temperature of thecomposition, no combustion residue, agglomerated in the form of abackbone of the pyrotechnic block, that is to say having the initialform of the pellet, is observed, as is customarily the case forcompositions based on BCN incorporating a high melting point refractoryoxide such as aluminum oxide.

1. A pyrotechnic gas-generating product, the composition of whichcomprises: guanidine nitrate, basic copper nitrate, and potassiumperchlorate, wherein said potassium perchlorate represents between 8%and 20% of the total weight of said pyrotechnic product; and saidcomposition additionally contains at least one oxide, chosen from metaloxides, metalloid oxides and mixtures thereof; said at least one oxiderepresenting between 1% and 5% of the total weight of said pyrotechnicproduct and having a melting point below the combustion temperature ofthe pyrotechnic product.
 2. The product as claimed in claim 1, whereinsaid potassium perchlorate represents between 10.5% and 20% of the totalweight of said pyrotechnic product.
 3. The product as claimed in claim1, wherein said melting point of said at least one oxide is at least 50K below said combustion temperature of said pyrotechnic product.
 4. Theproduct as claimed in claim 1, wherein said at least one oxide is chosenfrom silicon oxide (SiO₂), tungsten oxide (WO₃) and molybdenum oxide(MoO₃),
 5. The product as claimed in claim 4, wherein said at least oneoxide consists of silicon oxide.
 6. The product as claimed in claim 1,wherein its composition comprises, expressed as weight percentage: from50% to 65% of guanidine nitrate, from 20% to 40% of basic coppernitrate, from 8% to 20%, advantageously from 10.5% to 20%, of potassiumperchlorate, and from 1% to 5%, advantageously from 1% to 3%, of said atleast one oxide.
 7. The product as claimed in claim 1, wherein it isobtained by a dry process that comprises a step of compacting apulverulent mixture containing its constituent ingredients as powder,optionally followed by a step of granulation, itself followed, ifnecessary, by a step of forming via pelletizing.
 8. The product asclaimed in claim 1, wherein it is in the form of granules, pellets orblocks.
 9. A gas generator, suitable for an airbag, wherein it containsat least one product as claimed in claim
 1. 10. The product as claimedin claim 2, wherein said melting point of said at least one oxide is atleast 50 K below said combustion temperature of said pyrotechnicproduct.
 11. The product as claimed in claim 1, wherein its compositioncomprises, expressed as weight percentage: from 50% to 65% of guanidinenitrate, from 20% to 40% of basic copper nitrate, from 10.5% to 20% ofpotassium perchlorate, and from 1% to 5% of said at least one oxide. 12.The product as claimed in claim 11, wherein said at least one oxide ischosen from silicon oxide (SiO₂), tungsten oxide (WO₃) and molybdenumoxide (MoO₃).
 13. The product as claimed in claim 12, wherein said atleast one oxide consists of silicon oxide. from 20% to 40% of basiccopper nitrate, from 10.5% to 20% of potassium perchlorate, and from 1%to 5% of said at least one oxide.
 12. The product as claimed in claim11, wherein said at least one oxide is chosen from silicon oxide (SiO₂),tungsten oxide (WO₃) and molybdenum oxide (MoO₃).
 13. The product asclaimed in claim 12, wherein said at least one oxide consists of siliconoxide.